MOS (metal-oxide-semiconductor) varactors are semiconductor devices that have a capacitance that varies as a function of an applied voltage. Varactors are often used as tuning elements in circuits such as voltage controlled oscillators (VCOs), phase shifters, and phase locked loops (PLLs). For example, by varying a voltage applied to a varactor, the frequency of operation of an associated VCO can be adjusted.
The relationship between an applied voltage and a resulting capacitance is an important characteristic of a varactor. For example, the capacitance ratio, also known as the tunability, is a ratio of the highest and lowest capacitive values (Cmax/Cmin) that can be achieved by a varactor over a range of applied voltages. The capacitance ratio tells a designer how a capacitance of a varactor changes in response to an applied voltage.